A Twelve-week, Multi-center, Randomized, Double-blind, Double-dummy, Parallel-group, Active controlled, Escalating Dose Study to Compare the Effects on Sexual Functioning of Bupropion Hydrochloride Extended-release (WELLBUTRIN XL, 150-450 mg/day) and Extended-release Venlafaxine (EFFEXOR XR, 75-225 mg/day) in Subjects with Major Depressive Disorder